1. Field of the Invention
The present invention relates to particle beam metrology wherein a particle beam device such as a scanning electron microscope is used to perform critical dimension measurements of objects, for example integrated circuit wafers.
2. State of the Art
It is known to use electromagnetic systems in microscopes such as scanning electron microscopes (SEM) for measurement and inspection purposes. Scanning electron microscopes are often used in place of traditional optical microscopes for microelectronics inspection and metrology applications in semiconductor manufacturing. The metrology tools are often used, for example, for measuring patterns (i.e. critical dimensions) formed on semiconductor wafers during fabrication.
The short wavelengths of scanning electron microscopes have several advantages over conventionally used optical microscopes. For example, scanning electron microscopes can achieve resolutions from about 100 .ANG. to 200 .ANG., while the limit of resolution of optical microscopes is typically about 2500 .ANG.. Further, scanning electron microscopes provide depth of field several orders of magnitude greater than optical microscopes. Despite the accuracy and precision of present scanning electron microscopes, enhanced instrument specifications and capabilities are required as parameters (e.g. critical dimensions) to be inspected come within the sub-micrometer ranges.
An article entitled "Microelectronics Dimensional Metrology in the Scanning Electron Microscope", Parts I and II, Solid State Technology, by Michael T. Postek et al. (November 1986), describes a typical SEM wafer inspection system. As described therein, a focused electron beam is scanned from point to point on the specimen surface in a rectangular raster pattern. Accelerating voltage, beam current and spot diameter are optimized for the specific application and specimen composition.
As the scanning electron beam contacts the surface of the specimen, backscattered and/or secondary electrons are emitted from the specimen surface. Semiconductor inspection, analysis and metrology is performed by detecting the backscattered and/or secondary electrons. A point by point representation of the specimen is obtained on a CRT screen as the electron beam controllably scans the specimen.
Conventionally, a particle beam device such as a scanning electron microscope 7, shown in FIG. 1, includes a voltage source 11. The voltage source 11 is connected to an electron source 13 that directs a narrow beam of highly accelerated electrons towards a specimen stage 18 via a plurality of electron lenses L.sub.1, L.sub.2, and L.sub.3. The electron beam is indicated by the dashed line 14. The electron beam may be focused onto a wafer stage of the scanning electron microscope using an autofocus technique.
As further shown in FIG. 1, a cylindrical column 17 houses the electron source 13 and the lenses L.sub.1, L.sub.2, and L.sub.3. The column 17 is normally referred to as an electron optical column and includes a chamber, indicated in the drawing as 17A, that surrounds and supports a specimen stage 18. Together, the optical column 17 and the chamber 17A represent the body of the scanning electron microscope.
The scanning electron microscope 7 of FIG. 1 further includes an electrostatic deflection system for selectively scanning the electron beam across the specimen stage 18. The deflection system includes, for example, four pairs of electron beam scanning coils, designated D.sub.1 through D.sub.4. The scanning coils are located within optical column 17 for focusing the electron beam on the surface of the specimen held on stage 18. The pair of deflection coils D.sub.1 and D.sub.2 are connected to sawtooth voltage generator 19, and the pair of deflection coils D.sub.3 and D.sub.4 are connected to sawtooth voltage generator 20.
The electron beam scanning coils D.sub.1 through D.sub.4 deflect the electron beam 14 in two, generally perpendicular, directions. In the drawing, the deflection directions are designated as the x-direction and the y-direction, respectively. The x-direction and the y-direction typically are in a plane perpendicular to the direction of beam 14, but strict orthogonality is not required. For present purposes, it can be assumed that coils D.sub.1 and D.sub.3 deflect the scanning beam in the x-direction and that coils D.sub.2 and D.sub.4 deflect the scanning beam in the y-direction.
An electron collector 22 is arranged near the surface of a stage 18 which is exposed to beam 14. The electron collector is connected to an amplifier 23 which provides signals to an analog-to-digital converter 43 for transforming the collected electron current to digital signals which may be subsequently displayed on a video display 49.
In operation, saw-tooth generators 19 and 20 provide time-varying voltage signals to electron beam scanning coils D.sub.1 and D.sub.4 such that beam 14 is deflected across specimen stage 18 in a predetermined scanning pattern. The saw-tooth generators 19 and 20 typically operate synchronously to drive the electron beam across stage 18 in the x-direction at a constant rate, with each scan beam deflected in the y-direction to form a series of generally parallel scanning lines.
During operation of the FIG. 1 scanning electron microscope, collector 22 detects changes in the electron current at stage 18. Thus, as the electron beams scans a specimen on stage 18, changes in the composition, texture and topography of the specimen causing amplitude variations of the electron current detected by collector 22. With each complete scanning sequence, an image corresponding to features of the specimen can be created.
Traditional methods of electron imaging rely on secondary electron emission and, as a result, have limited capability for extracting information from the base of sub-micrometer contact holes and other high-aspect-ratio structures. Such a high-aspect-ratio contact hole is a common feature of semiconductor wafers and is shown in FIG. 2. A layer of resist 31 has been used to pattern a sub-micrometer contact hole in a layer of oxide 33 coated on the surface of a wafer 35. The height h of the contact hole is considerably greater than the width w of the contact hole such that h/w&gt;&gt;1. Typical values of h and w might be 2.0 .mu.m and 0.5 .mu.m respectively, giving an aspect ratio of 4:1.
The difficulty of imaging high-aspect-ratio structures using traditional methods of secondary electron imaging holds particularly true for structures with nearly vertical profiles. Because of the insulating properties of oxide and photoresist, directing a primary electron beam onto the wafer structure causes negative surface charge to accumulate at the surface of the photoresist layer as illustrated at FIG. 3. A few volts of surface potential can severely disturb the secondary electron image, since secondary electrons (SE) are typically emitted with energies less than 10 electron volts. In cases of severe charging, commonly encountered when imaging contact holes, secondary electron emission from the base may be shut off entirely. Attempts to control surface fields have been only partially effective and may themselves introduce artifacts by modulating the landing energy of the primary electron beam.
Charge accumulation on photoresist structures surrounding the features to be measured is a common cause of non-linearity and distortion not only in the case of contact holes but also in the case of photoresist lines and other features in general. Referring to FIG. 4A, as an electron beam is scanned across a photoresist line 71, charge accumulation on the surface of the photoresist line begins as the beam contacts the surface of the photoresist line and continues at a constant rate as the beam traverses the photoresist line such that, by the time the beam leaves the surface of the photoresist line, a significant amount of charge has accumulated, resulting in a baseline shift of the resulting signal.
Conventional scanning techniques have only contributed to the problem of charge accumulation. When a measurement is to be performed of a feature in an image field, conventionally, the entire image field is scanned. For example, referring to FIG. 5A, measurements may be required of the contact hole 73, the isolated line 75, and the dense line 77. Despite these features occupying only a portion of the image field 79, conventionally, the entire image field is scanned. As a result, scan accumulation occurs even in areas where no information is being generated, adversely affecting accuracy in those areas where information is being generated.
In the prior art, contact holes, as with other object features, have been raster scanned as shown in FIG. 6A. In addition to surface charging, raster scanning of contact holes has another significant drawback. To facilitate detection of the edge of the contact hole, the "attack angle" of the particle beam to the contact hole should ideally be perpendicular. In FIG. 6A, this ideal attack angle is approximated only near the horizontal radius of the contact hole. For scans above and below this medial line, the attack angle becomes smaller and smaller (and thus poorer and poorer) as the distance from the medial line increases. Edge detection therefore becomes difficult.
Accordingly, a need exists for improved imaging techniques for extracting information from the base of sub-micrometer contact holes and other high-aspect-ratio structures. A further need exists for methods of reducing the distortion effects of baseline drift during scanning of photoresist structures. In general, a need exists for improved scanning methods that reduce the deleterious effects of surface charge accumulation, improving precision and linearity.